Method and apparatus for detecting and decoding data in a PRML class-IV digital communication channel

ABSTRACT

A method and apparatus for encoding, detecting and decoding data in a Partial Response (PR) class-IV magnetic recording storage system that does not require the conventional interleave constraint and therefore minimizes the path memory and latency of a sequence detector such as the Maximum Likelihood (ML) Viterbi detector. Rather than encoding an interleave constraint to ensure merging of path memories in the detector, the parity of the encoded codewords is utilized in selecting tile correct sequence out of the unmerged paths. The encoding technique encodes the data using two groups of codewords; the first group causes the path memories of the sequence detector to merge into one survivor sequence and the second group causes the path memories to merge into two survivor sequences different in only one bit and thus different in parity. The correct survivor sequence is thereby selected according to the parity of the codeword being detected. Parity is determined during the encoding process such that codewords from the second group follow codewords with even or odd parity. By looking ahead to the following codeword during the detection process, the parity of the current codeword being detected is determined and the correct survivor sequence selected. Further, the encoding technique provides the ability to correct for certain errors in the detected sequence when a &#34;1&#34; bit is erased due to noise in the system.

FIELD OF INVENTION

This invention related to computer technology and more specifically to systems for storing and retrieving digitized data on magnetic storage media.

CROSS REFERENCE TO RELATED APPLICATIONS AND PATENTS

This application is related to U.S. patent application Ser. No. 08/258,308 entitled, "Method and Apparatus for Encoding Data in a PRML Class-IV Digital Communication Channel". The above-named patent application is assigned to the same entity.

BACKGROUND OF THE INVENTION

The application of Partial Response Maximum Likelihood (herein after PRML) techniques to digital communication channels is well documented. See Y. Kabal and S. Pasupathy, "Partial Response Signaling", IEEE Trans. Commun. Tech., Vol. COM-23, pp.921-934, Sep. 1975; Edward A. Lee and David G. Messerschmitt, "Digital Communication", Kluwer Academic Publishers, Boston, 1990; and G. D. Forney, Jr., "The Viterbi Algorithm", Proc. IEEE, Vol. 61, pp. 268-278, Mar. 1973. Applying PRML techniques to magnetic storage systems is also well documented. See Roy D. Cideciyan, Francois Dolivo, Walter Hirt, and Wolfgang Schott, "A PRML System for Digital Magnetic Recording", IEEE Journal on Selected Areas in Communications, Vol. 10 No. 1, January 1992, pp.38-56; Wood et al, "Viterbi Detection of Class IV Partial Response on a Magnetic Recording Channel", IEEE Trans. Commun., Vol. Com-34, No. 5, pp. 454-461, May 1986; and Coker Et al, "Implementation of PRML in a Rigid Disk Drive", IEEE Trans. on Magnetics, Vol. 27, No. 6, November 1991.

In magnetic storage systems, a transducing head writes digital data onto the surface of a magnetic storage medium, such as a magnetic disk. The digital data serves to modulate the current in the head coil in writing a sequence of corresponding magnetic flux transitions which represent the digital data. When reading the recorded data, the head again passes over the magnetic medium and transduces the magnetic transitions into pulses in an analog read signal, which are then decoded by read channel circuitry to reproduce the digital sequence.

Decoding the pulses into a digital sequence can be performed by a simple pulse detector read channel or, as in more recent designs, by a partial response maximum likelihood (PRML) read channel. The PRML read channel scheme is preferred over the simpler pulse detection scheme because it decreases the necessary bandwidth, thereby allowing more data to be stored on the storage medium.

In conventional peak detection as well as PRML schemes, a channel code provides clocking and automatic gain control (AGC) information. To perform the timing and gain control, the number of consecutive zero samples must be limited since timing and in control information is derived only when non-zero samples are read from the channel. Typically, an RLL code having a (d,k) constraint, which specifies the minimum and maximum run lengths of zeros respectively, encodes the data before recording it to the storage medium. For instance, a byte oriented (d=0,k=4) 8/9 encodes binary data represented as 8 bit data bytes into 9 bit codewords in order to achieve the desired (d,k) constraint. Because the PRML technique utilizes inter-symbol interference (ISI) in a controlled manner, the d constraint in Partial Response class-IV channels is unnecessary (d=0). However the k constraint is still necessary in class-IV systems to provide the timing and control information.

The sampled data in Partial Response (PR) channels is typically converted into digital sequence using Maximum Likelihood (ML) detection techniques (thus PRML). A Viterbi sequence detector normally implements the maximum likelihood Viterbi algorithm for detecting the digital sequence from the sampled data. See G. D. Forney, Jr., "The Viterbi Algorithm", Proc. IEEE, Vol. 61, pp. 268-278, Mar. 1973.

Prior art implementations of encoders and Viterbi sequence detectors in Partial Response class-IV channels introduced an additional constraint on the number of consecutive zeros in each of the even an odd interleaves of the encoded data in order to minimize the path memory of the detector. Thus the conventional modulation codes for PR class-IV systems utilizing Viterbi detection are characterized by (d,k/k1) where k1 represents the maximum run length of zeroes in both the even and odd subsequences. A small value of k is desirable for accurate timing and gain control, and a small value of k1 reduces the size of the path memory in the Viterbi detector. A method for encoding the channel data according to the (d,k/k1) constraints is described in U.S. Pat. No. 4,707,681, the disclosure of which is herein incorporated by reference.

FIG. 3 shows an example trellis diagram for the 1-D² PR class-IV channel and the affect the k1 constraint has on the path memory length of the Viterbi detector. As shown in the diagram, the path memories merge into one survivor sequence only after a "1" has been detected in both the even 40 and odd 42 interleaves. Therefore, in prior art implementations the length of the Viterbi detector path memories must be greater than 2(k1+1)+1 to guarantee that the paths merge into the correct sequence 34.

A further problem with the prior art implementations is that a minimum delay of 2(k1+1)+1 samples is required before the paths merge and the correct sequence is available. This latency degrades the performance of the storage systems; for instance, it increases the delay between reading the ID field and writing data to the disk. The latency also degrades the performance of other read channel components that use the output of the sequence detector, such as servo control.

Another problem with the prior art is the inability to correct errors in the data samples due to noise. For instance, if noise in the system causes a "1" to be erased, the path memories of the Viterbi detector may not be merged after 2(k1+1)+1 samples and the detected sequence may not be correct.

It is a general object of the present invention to provide a method and apparatus for processing data in a PR class-IV communication channel that does not require the conventional, k1 constraint, thereby minimizing the path memory and latency of the Viterbi detector. Another object of the invention is to correct errors in the detected sequence when a "1" is erased due to noise in the system.

SUMMARY OF THE INVENTION

The objects of the present invention are achieved by a novel method and apparatus for encoding, detecting and decoding data in a PR class-IV magnetic recording channel. The data is encoded using run length limited (RLL) (d,k) coding, without the conventional k1 constraint (i.e., there is no interleave constraint on the maximum number of consecutive zeros). There are two preferred embodiments of the present invention; the first embodiment uses minimum path memory and minimizes the latency of the Viterbi detector but does not correct for errors due to noise, and the second embodiment corrects for errors but uses more path memory and has more latency.

In the minimum latency embodiment, the codewords are divided into three sets. The first set contains codewords having at least one "1" bit in both the even and odd interleave within a predetermined first N number of bits. The second set contains codewords having at least one "1" bit in the even interleave and zero "1" bits in the odd interleave within the first N number of bits. The third set contains codewords having at least one "1" bit in the odd interleave and zero "1" bits in the even interleave within the first N number of bits.

The path memories of the minimum latency embodiment are N+n long where n is the num her of bits in a codeword (e.g., 9 in a 8/9 code). Thus the path memories are long enough to store N bits out of a current codeword, and n (i.e. all) bits of the previous codeword. Because the codewords in the first set contain a "1" in both interleaves within the first N bits, the path memories prior to the two "1" bits are guaranteed to have merged. That is, the codeword detected previous to the N bits of the current codeword is guaranteed to be available in the merged part of the path memories as shown in FIG. 5a.

The codewords in the second set, having at least one "1" bit in the even interleave within the first N bits of the codeword, guarantees that the path memories storing the even interleave of the previous codeword will be the same and the odd interleave will be different in only one bit as shown in FIG. 5b. Similarly, the codewords in the third set, having at least one "1" bit in the odd interleave within the first N bits of the codeword, guarantees that the path memories storing the odd interleave of the previous codeword will be the same and the even interleave will be different in only one bit as shown in FIG. 5c.

Since the path memories storing the previous codeword are different in only one bit when followed by a codeword from either the second or third set, the correct path can be selected by knowing the parity of the previous codeword as determined by which set the following codeword belongs. For instance, if codewords from the second set always follow codewords with even parity, and codewords from the third set always follow codewords wit odd parity, the correct sequence for the previous codeword can be determined by choosing the path with the correct parity. The encoding method and apparatus of the present invention ensures that codewords with an even or odd parity are followed by codewords from the correct set.

By encoding the data according to the present invention, the correct sequence for the previous codeword can be determined before the path memories merge, thereby minimizing the latency of the Viterbi detector. Compared to the prior art implementations having a latency of 2(k1+1)+1 bits, the latency of the present invention is N bits. Thus if K1=4 and N=3, the latency of the present invention is approximately four times less than the prior art.

In the error correcting embodiment of the present invention, the path memories are long enough to store M+n bits, where M is greater than N in the first embodiment. The codewords are again divide into three sets. The first set has an additional property of at least two "1" bits in both the even and odd interleaves within a predetermined first M bits of the codeword. The second and third set have an additional property of at least one "1" bit in one of the interleaves and less than two "1" bits in the other interleave within the first M bits of the codeword.

If the codeword out of the second or third set has only one "1" bit in either interleave within the M bits, and that "1" bit is erased, the path memories storing the previous codeword will not have merged. However, since the parity of the previous codeword is known, the correct path can still be chosen and the errors caused by choosing the wrong path in the prior art implementations avoided.

Two example codes corresponding to the two embodiments are disclosed to illustrate the encoding technique of the present invention. Both are 8/9 rate (0,4) codes meaning that each eight source bits encoded into nine channel bits such that no more than four consecutive "0"s occur in the encoded data. There is no k1 constraint, that is, there is no constraint on the maximum number of consecutive "0"s in either interleave.

The two example codes are not meant to limit the invention. Codes having rates other than 8/9 and constraints other than (0,4) could be used without departing from the scope and spirit of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the recording system including an encoder, a pre-coder, a 1-D² Partial Response class-IV channel, a Viterbi detector and a decoder.

FIG. 2 shows a state diagram for a 1-D² Partial Response Class-IV channel and the corresponding survivor sequence extension from each state for the trellis diagram.

FIG. 3 is an example trellis diagram for a 1-D² Partial Response Class-IV channel as related to the prior art k1 constraint.

FIG. 4 is a block diagram of the encoder of the present invention.

FIG. 5a, 5b, and 5c show example trellis diagrams for a 1-D² Partial Response Class-IV channel storing a first and second codeword of the present invention.

FIG. 6a, 6b, and 6c show example trellis diagrams for a pair of 1-D channels processing the even and odd interleaves of the 1-D² Partial Response Class-IV channel storing a first and second codeword of the present invention.

FIG. 7a and 7b show example trellis diagrams for a 1-D² Partial Response Class-IV channel storing a first and second codeword of the present invention where a "1" bit has been erased from the second codeword due to noise.

FIG. 8a, 8b and 8c show example trellis diagrams for a 1-D² Partial Response Class-IV channel storing a first and second codeword of the present invention where a look-ahead signal is generated from the path memories storing the first bit of the second codeword.

FIG. 8d is a block diagram of the detector/decoder of the present invention where the detector is implemented using a pair of two state 1-D sliding threshold Viterbi detectors.

FIG. 9a and 9b show a detailed diagram of the two state 1-D sliding threshold Viterbi detector for the minimum latency embodiment of the present invention.

FIG. 10 shows the implementation of the multiplexor for selecting the output of the two state 1-D sliding threshold Viterbi detectors.

FIG. 11a and 11b show a detailed diagram of the two state 1-D sliding threshold Viterbi detector for the error correcting embodiment of the present invention.

FIG. 12 shows a block diagram of the four state 1-D² Viterbi detector for an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Partial Response class-IV magnetic recording channels are characterized by a 1-D² polynomial which exhibits spectral nulls at dc and at the Nyquist frequency. The output of the channel is thus a function of the current input and the input at n-2:

    y(n)=x(n)-x(n-2).

With the binary input data modulating the write current such that a "1" bit generates current in a positive direction (+1) and a "0" bit in the negative direction (-1), the output samples of the channel y(n) take on the values 2,0,-2, hereinafter normalized to 1 0,-1.

Referring now to FIG. 1, the 8-bit data words are encoded into 9-bit codewords using the encoder 3 of the present invention. In order to prevent error propogation and to retain the global k constraint for timing and gain control, the output of the encoder 20 is pre-coded by a 1/1-D² polynomial 2 before writing the codewords to the channel 6. This pre-coding is taken into account in the Viterbi detector 4 when the sampled data is read from the recording channel 6. The detector 4 detects the digital sequence for the codewords and the codewords are decoded 5 back into 8-bit data words.

The binary input 16 the channel 6 and normalized output 18 can be depicted by a state diagram 8 as shown in FIG. 2. In the diagram a state 12 represents the previous two output bits 16 from the pre-coder 2. A transition is depicted by the current encoder output 20 and the corresponding pre-coder output 16 and channel output 18.

The operation of the Viterbi sequence detector can be further understood from a trellis diagram. As shown in FIG. 2, each state of the state diagram 8 corresponds to a state in the trellis diagram 10. Each path 22 from one state to the next represents one bit 16 of a survivor sequence for a detected codeword given the current sample value 18 read from the 1-D² channel 6. A survivor sequence is a possible correct recorded sequence given several samples read from the channel.

For example, a non-zero sample read from the channel is caused either by a "1" in the recorded sequence or a "0" in the recorded sequence and noise in the channel. Thus two survivor sequences are saved, one with a "1" and the other with a "0". The correct sequence is determined from future samples read from the channel; that is, the correct sequence is determined by choosing the sequence with the maximum likelihood of being correct given future samples read from the channel. The maximum likelihood sequence is the sequence that minimizes the mean squared error between the sample value read from the channel and the sample value expected. For instance, if a sample value read from the channel is +0.8, then the squared sample error value for a "0" in the sequence is (0-(+0.8))², and the squared sample error value for a "1" in the sequence is (+1-(+0.8)².

A sum of the mean squared errors is stored for each survivor sequence, and the sequence with the least mean squared error has the highest probability of being correct. After a "1" in each interleave of the recorded sequence has been processed by the Viterbi detector, there is only one survivor sequence with a minimum mean squared error up to the first "1", and the paths storing the other survivor sequences will have merged into this maximum likelihood path.

At any given time there can be up to four survivor sequences; there are four path memories in the Viterbi detector. To aid in understanding the invention, each of the four path memories is represented by a corresponding state in the trellis diagram.

Referring now to FIG. 3, shown is a trellis diagram for a particular recorded sequence 36 (i.e., an encoded sequence) and corresponding sample sequence 38 read from the 1-D² channel 6. The sequence is labeled as odd 30 and even 32 interleaves, and each interleave has a constraint of k1=4; that is, the encoding scheme uses the prior art technique. As shown in the diagram, the path memories do not merge into a single survivor sequence 34 until a "1" bit 40 in the even interleave and a "1" bit 42 in the odd interleave have been detected. Therefore the length of the path memories must be at least 2(k1+1)+1 or 2(4+1)+1=11 bits long, and the next bit in the correct sequence available until 11 samples have been read from the channel. Before a complete codeword is available, there is a latency of 2(k1+1)+1+n, where n is the number of bits in a codeword (e.g., 9 in a 8/9 code). This latency is undesirable as previously described. Further, if the "1" bit 42 in the odd interleave is erased, the path memories will not have merged after 2(k1+1) bits and the survivor sequence selected for the codeword may not be correct.

ENCODER

A block diagram of the encoder 3 of the present invention is shown in FIG. 4. Eight bits 48 of source data are encoded 50 into nine bit codewords 20 using the parity 54 (modulo-2 sum) of the previous codeword. The encoding scheme is unique to the present invention and is hereinafter described.

The minimum latency encoding scheme of the present invention, which has the k constraint but not the k1 constraint, can be understood from the trellis diagrams shown in FIGS. 5a, 5b and 5c. In these figures, an 8/9 code is implemented and the trellis diagram stores 9 bits 60 of a first codeword and 2 bits 62 from a second codeword that follows the first.

As shown in FIG. 5a, if the 2 bits 62 from the second codeword are {1,1}, then the path memories storing the first codeword will have merged into a single survivor sequence 60 because there is a "1" bit in both the even and odd interleave. That is, the parity of the first codeword is not necessary to select the correct path memory because all the path memories are the same If the 2 bits 62 from the second codeword are {0,1} or {1,0}, then the path memories will store two survivor sequences different by only one bit.

For instance, FIG. 5b shows the case where the 2 bits 62 from the second codeword are {0,1} wherein the two resulting survivor sequences for the first codeword are {1,1,0,0,0,0,0,0,0} 64 and {1,0,0,0,0,0,0,0,0} 66. Since the two sequence are different in only one bit, the correct sequence can be selected using the parity of the first codeword. Thus if the parity is even, the first survivor sequence 64 is selected, and if the parity is odd, the second survivor sequence 66 is selected. FIG. 5c shows the same result for the case where the 2 bits 62 from the second codeword are {1,0}. The parity of the first codeword is determined during the encoding process.

If the second data word encodes into a codeword out of a first group of codewords wherein the first two bits are {1,1}, then the parity of the first codeword is ignored since the path memories will have merged into one survivor sequence. If the second data word does not encode into the {1,1} group, then it is encoded into either the {0,1} or {1,0} group of codewords depending on the parity of the first codeword. For instance, if the first codeword has even parity, then the second codeword is selected from the {0,1} group of codewords, and if the first codeword has odd parity, then the second codeword is selected from the {1,0} group. When the data is detected by the Viterbi detector, the path memory storing the first codeword is selected based on its corresponding parity as determined by the second codeword.

To this point, the invention has been described with respect to a 1-D² four state Viterbi detector. However, the 1-D² channel can be factored into a pair of interleaved 1-D channels and detected using a pair of two state Viterbi detectors. See Roy D. Cideciyan, Francois Dolivo, Walter Hirt, and Wolfgang Schott, "A PRML System for Digital Magnetic Recording", IEEE Journal on Selected Areas in Communications, Vol. 10 No 1, January 1992, pp.38-56. Using two interleaved sequence detectors simplifies the detection algorithm, and the encoding technique of the present invention is applicable to this interleaved detection scheme.

FIGS. 6a, 6b, and 6c show the two state trellis diagrams for the interleaved detectors corresponding to the four state trellis diagrams in FIGS. 5a, 5b, and 5c, respectively. In FIG. 6a there is a "1" in both interleaves within the first two bits of the second codeword; the path memories for both the even and odd interleave have merged into the correct sequence for the first codeword. In FIG. 6b there is a "1" only in the odd interleave within the first two bits of the second codeword; the even interleave has not merged and contains two survivor sequences. FIG. 6c shows a similar situation where the odd interleave has not merged into a single survivor sequence.

As in the four state detector described above, the correct survivor sequence for the first codeword is selected based on its parity as determined by the second codeword. The even interleave will not be merged only when followed by a codeword from the {1,0} group; the path memory of the even interleave is selected such that the parity of the first codeword is odd since codewords that precede the {1,0} codewords always have odd parity. Similarly, the odd interleave will not be merged only when preceded by a codeword from the {0,1} group; the path memory of the odd interleave is selected such that the parity of the first codeword is even since codewords that precede the {0,1} codewords always have even parity.

For the 8/9 code having a {0,4} constraint, there are not enough 9 bit codewords in the groups of {1,1,X,X,X,X,X,X,X}, {0,1,X,X,X,X,X,X,X}, and {1,0,X,X,X,X,X,X,X} to encode 256 data words; a third bit must be used. The resulting groups of codewords and corresponding parity of the previous codeword are shown in Table 1.

                  TABLE 1                                                          ______________________________________                                         GROUP1: {0,0,1,X,X,X,X,X,X}                                                                            odd parity                                             GR0UP2: {0,1,0,X,X,X,X,X,X}                                                                            odd parity                                             GR0UP3: {0,1,1,X,X,X,X,X,X}                                                                            don't care                                             GR0UP4: {1,0,0,X,X,X,X,X,X}                                                                            even parity                                            GR0UP5: {1,0,1,X,X,X,X,X,X}                                                                            even parity                                            GR0UP6: {1,1,0,X,X,X,X,X,X}                                                                            don't care                                             GR0UP7: {1,1,1,X,X,X,X,X,X}                                                                            don't care                                             ______________________________________                                    

Groups 3, 5, 6 and 7 have "1" bit in both the even and odd interleave within the firs 3 bits and thus perform the same function as the {1,1} group described above. The remaining groups are divided into those that follow codewords with even and odd parity as follow; groups 1 and 2 always follow codewords with odd parity, and groups 4 and 5 always follow codewords with even parity. The mapping of data words to codewords is arbitrary, and the assignment of even and odd parity may also be reversed.

An example mapping octal notation of data words to codewords is shown in Table 2 and Table 3. Table 2 shows the mapping of data words into codewords out of groups 3, 5, 6 and 7. Table 3 shows the mapping of data words into codewords out of groups 1 and 2 if the parity of the previous codeword is odd, and into codeword out of groups 4 and 5 if the parity of the previous codeword is even.

                  TABLE 2                                                          ______________________________________                                                 Data CW                                                                ______________________________________                                                 000  362                                                                       001  342                                                                       002  312                                                                       003  352                                                                       004  662                                                                       005  642                                                                       006  612                                                                       007  652                                                                       010  332                                                                       011  322                                                                       012  311                                                                       013  313                                                                       014  341                                                                       015  343                                                                       016  351                                                                       017  353                                                                       020  304                                                                       021  306                                                                       022  314                                                                       023  316                                                                       024  344                                                                       025  346                                                                       026  354                                                                       027  356                                                                       030  305                                                                       031  307                                                                       032  315                                                                       033  317                                                                       034  345                                                                       035  347                                                                       036  355                                                                       037  357                                                                       040  703                                                                       041  742                                                                       042  712                                                                       043  752                                                                       044  762                                                                       045  372                                                                       046  672                                                                       047  772                                                                       050  321                                                                       051  323                                                                       052  331                                                                       053  333                                                                       054  361                                                                       055  363                                                                       056  371                                                                       057  373                                                                       060  324                                                                       061  326                                                                       062  334                                                                       063  336                                                                       064  364                                                                       065  366                                                                       066  374                                                                       067  376                                                                       070  325                                                                       071  327                                                                       072  335                                                                       073  337                                                                       074  365                                                                       075  367                                                                       076  375                                                                       077  377                                                                       110  632                                                                       111  622                                                                       112  611                                                                       113  613                                                                       114  641                                                                       115  643                                                                       116  651                                                                       117  653                                                                       120  604                                                                       121  606                                                                       122  614                                                                       123  616                                                                       124  644                                                                       125  646                                                                       126  654                                                                       127  656                                                                       130  605                                                                       131  607                                                                       132  615                                                                       133  617                                                                       134  645                                                                       135  647                                                                       136  655                                                                       137  657                                                                       150  621                                                                       151  623                                                                       152  631                                                                       153  633                                                                       154  661                                                                       155  663                                                                       156  671                                                                       157  673                                                                       160  624                                                                       161  626                                                                       162  634                                                                       163  636                                                                       164  664                                                                       165  666                                                                       166  674                                                                       167  676                                                                       170  625                                                                       171  627                                                                       172  635                                                                       173  637                                                                       174  665                                                                       175  667                                                                       176  675                                                                       177  677                                                                       210  732                                                                       211  722                                                                       212  711                                                                       213  713                                                                       214  741                                                                       215  743                                                                       216  751                                                                       217  753                                                                       220  704                                                                       221  706                                                                       222  714                                                                       223  716                                                                       224  744                                                                       225  746                                                                       226  754                                                                       227  756                                                                       230  705                                                                       231  707                                                                       232  715                                                                       233  717                                                                       234  745                                                                       235  747                                                                       236  755                                                                       237  757                                                                       250  721                                                                       251  723                                                                       252  731                                                                       253  733                                                                       254  761                                                                       255  763                                                                       256  771                                                                       257  773                                                                       260  724                                                                       261  726                                                                       262  734                                                                       263  736                                                                       264  764                                                                       265  766                                                                       266  774                                                                       267  776                                                                       270  725                                                                       271  727                                                                       272  735                                                                       273  737                                                                       274  765                                                                       275  767                                                                       276  775                                                                       277  777                                                               ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                                        CW      CW                                                                     even    odd                                                     Data           parity  parity                                                  ______________________________________                                         100            542     142                                                     101            503     103                                                     102            511     111                                                     103            513     113                                                     104            541     141                                                     105            543     143                                                     106            551     151                                                     107            553     153                                                     140            532     132                                                     141            523     123                                                     142            531     131                                                     143            533     133                                                     144            561     161                                                     145            563     163                                                     146            571     171                                                     147            573     173                                                     200            412     232                                                     201            442     222                                                     202            411     211                                                     203            413     213                                                     204            441     252                                                     205            443     243                                                     206            451     251                                                     207            453     253                                                     240            432     221                                                     241            423     223                                                     242            431     231                                                     243            433     233                                                     244            461     261                                                     245            463     263                                                     246            471     271                                                     247            473     273                                                     300            552     152                                                     301            506     106                                                     302            514     114                                                     303            516     116                                                     304            544     144                                                     305            546     146                                                     306            554     154                                                     307            556     156                                                     310            522     122                                                     311            507     107                                                     312            515     115                                                     313            517     117                                                     314            545     145                                                     315            547     147                                                     316            555     155                                                     317            557     157                                                     320            562     162                                                     321            526     126                                                     322            534     134                                                     323            536     136                                                     324            564     164                                                     325            566     166                                                     326            574     174                                                     327            576     176                                                     330            525     172                                                     331            527     127                                                     332            535     135                                                     333            537     137                                                     334            565     165                                                     335            567     167                                                     336            575     175                                                     337            577     177                                                     340            572     272                                                     341            422     262                                                     342            414     214                                                     343            416     216                                                     344            444     244                                                     345            446     246                                                     346            454     254                                                     347            456     256                                                     350            452     205                                                     351            512     207                                                     352            415     215                                                     353            417     217                                                     354            445     245                                                     355            447     247                                                     356            455     255                                                     357            457     257                                                     360            462     224                                                     361            426     226                                                     362            434     234                                                     363            436     236                                                     364            464     264                                                     365            466     266                                                     366            474     274                                                     367            476     276                                                     370            472     225                                                     371            427     227                                                     372            435     235                                                     373            437     237                                                     374            465     265                                                     375            467     267                                                     376            475     275                                                     377            477     277                                                     ______________________________________                                    

The encoder 3 can be implemented with either combinatorial logic or sequential logic or can be implemented as a look-up table, e.g. a ROM. A person skilled in the art could also implement the encoding scheme in software without departing from the scope and spirit of the invention. In the preferred embodiment the encoder is implemented with combinatorial logic and tile logic equations are listed in Table 4 where "abcdefgh" is the input byte, p is the parity of the previous codeword, and "ABCDEFGHJ" is the output codeword. An exclamation! indicates an inverted term. "A" and "a" are the most significant bits of the input byte and output codeword, respectively.

                  TABLE 4                                                          ______________________________________                                         A =   !a !b !d !e f g + !a !b !d !e f !h + a !p                                      + b !p + !a !b c !d !e !f + !a !b !c !d !e f                                   + a !b d + !a b d +a !b e + !a b e.                                      B =   a c p + !b p + !a d + !a e + !a !b + !b d + !b e.                        C =   !a !b c !g + !a !b c h + a b !c + !a !b !f + !a b !d !e +                      a !d e !f !g h !p + b !d !e !f !g !h !p + !b d + !b e.                   D =   a b !c !e !g !h + !a !c !d !e !g !h + a b c !g !h !p +                         a b !c d !g !h p + !b !c !d !e !g h !p + a b c !d !e !g p +                    !a !b !d !e h + a f + b f + e f + d f + c f.                             E =   !c !d e !f !g !h + !a !b !c !d !e !g !h + b c !e !f !g +                       !b c e + !b c f + a !b !d !f !g p + a b d + !a b c +                           a !b c +!a !d e !f !g + !b !d e !f !g + c d.                             F =   a !c !d !e !f !h + !b !c !d e !f !h + a b !c d e !f !h p +                     a !b !c !d !e !h p + a b c e !f !h !p + b c !d !e !f !h +                      a !d !e !f !h !p + a b c !d e !f !p + !a !b c !d !e f h +                      !a !c !d e !f !h + g.                                                    G =   a b c e p + a b !c h + a b f + a b g + !b d + c d p +                          !c d e !p + d h + !a d.                                                  H =   a b !e !f !g !p + a b c !f !g !p + a b !c !f !g p +                            !b !c !d !e !g p + b !d !e !f !g + !d !e !f !g !p +                            !c !d !f !g + h + !a !b !d !e.                                           J =   a !b c !d p + !a b !d !e h + a !b !d f !p + a !b c !d h +                      a !b !d g + !a b !d f + !a b !d g + a !b !d f h +                              !c d e !p + d e h + c e p + e f + e g + a b !c e h +                           !a !b c !d !f !g !h + !b d e + !a d e + !a c e + !b c                    ______________________________________                                               e.                                                                  

The error correcting encoding technique of the present invention, which has the k constraint but not the k1 constraint, can be understood from the trellis diagrams shown in FIGS. 7a and 7b. In these figures, an 8/9 code is implemented and the trellis diagram stores 9 bits 70 of a first codeword and 9 bits 72 from a second codeword that follows the first. If the 9 bits 72 from the second codeword has at least one "1" bit in one interleave and only one "1" bit in the other interleave and that "1" bit in the other interleave is erased due to noise, then the path memories will store two survivor sequences different by only one bit.

For instance, FIG. 7a shows the case wherein a "1" bit 78 in the odd interleave of the second codeword has been erased due to noise. The two resulting survivor sequences for the first codeword are {1,0,0,0,0,0,0,0,0} 80 and {1,0,0,0,1,0,0,0,0} 82. Since the two sequence are different in only one bit, the correct sequence can be selected using the parity of the first codeword. Thus if the parity is odd, the first survivor sequence 80 is selected, and if the parity is even, the second survivor sequence 82 is selected. FIG. 7b shows the same result for the case wherein a "1" bit 84 in the even interleave of the second codeword has been erased due to noise. As in the minimum latency encoding scheme, the parity of the first codeword is determined during the encoding process.

An example mapping in octal notation of data words to codewords is shown in Table 5 and Table 6. Table 5 shows the mapping of data words into codewords out of all groups 1, 2, 3, 4, 5, 6 and 7 which have at least two "1" bits in both the even and odd interleaves. Table 6 shows the mapping of data words into codewords out of groups 1 and 2 if the parity of the previous codeword is odd, and into codewords out of groups 4 and 5 if the parity of the previous codeword is even. The codewords in Table 6 have zero or one "1" bit in one interleave and at least one "1" bits in the other interleave.

                  TABLE 5                                                          ______________________________________                                                 Data CW                                                                ______________________________________                                                 000  416                                                                       001  446                                                                       002  454                                                                       003  456                                                                       004  436                                                                       005  466                                                                       006  474                                                                       007  476                                                                       010  113                                                                       011  143                                                                       012  151                                                                       013  153                                                                       014  513                                                                       015  543                                                                       016  551                                                                       017  553                                                                       020  116                                                                       021  146                                                                       022  154                                                                       023  156                                                                       024  516                                                                       025  546                                                                       026  554                                                                       027  556                                                                       030  117                                                                       031  147                                                                       032  155                                                                       033  157                                                                       034  517                                                                       035  547                                                                       036  555                                                                       037  557                                                                       040  136                                                                       041  166                                                                       042  174                                                                       043  176                                                                       044  536                                                                       045  566                                                                       046  574                                                                       047  576                                                                       050  133                                                                       051  163                                                                       052  171                                                                       053  173                                                                       054  533                                                                       055  563                                                                       056  571                                                                       057  573                                                                       061  226                                                                       062  234                                                                       063  236                                                                       064  264                                                                       065  266                                                                       066  274                                                                       067  276                                                                       071  227                                                                       072  235                                                                       073  237                                                                       074  265                                                                       075  267                                                                       076  275                                                                       077  277                                                                       100  433                                                                       101  463                                                                       102  471                                                                       103  473                                                                       104  437                                                                       105  467                                                                       106  475                                                                       107  477                                                                       111  303                                                                       112  311                                                                       113  313                                                                       114  341                                                                       115  343                                                                       116  351                                                                       117  353                                                                       121  306                                                                       122  314                                                                       123  316                                                                       124  344                                                                       125  346                                                                       126  354                                                                       127  356                                                                       131  307                                                                       132  315                                                                       133  317                                                                       134  345                                                                       135  347                                                                       136  355                                                                       137  357                                                                       140  271                                                                       141  132                                                                       142  162                                                                       143  172                                                                       144  322                                                                       145  332                                                                       146  362                                                                       147  372                                                                       151  323                                                                       152  331                                                                       153  333                                                                       154  361                                                                       155  363                                                                       156  371                                                                       157  373                                                                       161  326                                                                       162  334                                                                       163  336                                                                       164  364                                                                       165  366                                                                       166  374                                                                       167  376                                                                       171  327                                                                       172  335                                                                       173  337                                                                       174  365                                                                       175  367                                                                       176  375                                                                       177  377                                                                       200  413                                                                       201  443                                                                       202  451                                                                       203  453                                                                       204  417                                                                       205  447                                                                       206  455                                                                       207  457                                                                       211  263                                                                       212  611                                                                       213  613                                                                       214  641                                                                       215  643                                                                       216  651                                                                       217  653                                                                       221  606                                                                       222  614                                                                       223  616                                                                       224  644                                                                       225  646                                                                       226  654                                                                       227  656                                                                       231  607                                                                       232  615                                                                       233  617                                                                       234  645                                                                       235  647                                                                       236  655                                                                       237  657                                                                       240  233                                                                       241  432                                                                       242  462                                                                       243  472                                                                       244  622                                                                       245  632                                                                       246  662                                                                       247  672                                                                       251  623                                                                       252  631                                                                       253  633                                                                       254  661                                                                       255  663                                                                       256  671                                                                       257  673                                                                       261  626                                                                       262  634                                                                       263  636                                                                       264  664                                                                       265  666                                                                       266  674                                                                       267  676                                                                       271  627                                                                       272  635                                                                       273  637                                                                       274  665                                                                       275  667                                                                       276  675                                                                       277  677                                                                       300  137                                                                       301  167                                                                       302  175                                                                       303  177                                                                       304  537                                                                       305  567                                                                       306  575                                                                       307  577                                                                       311  703                                                                       312  711                                                                       313  713                                                                       314  741                                                                       315  743                                                                       316  751                                                                       317  753                                                                       321  706                                                                       322  714                                                                       323  716                                                                       324  744                                                                       325  746                                                                       326  754                                                                       327  756                                                                       331  707                                                                       332  715                                                                       333  717                                                                       334  745                                                                       335  747                                                                       336  755                                                                       337  757                                                                       340  273                                                                       341  532                                                                       342  562                                                                       343  572                                                                       344  722                                                                       345  732                                                                       346  762                                                                       347  772                                                                       351  723                                                                       352  731                                                                       353  733                                                                       354  761                                                                       355  763                                                                       356  771                                                                       357  773                                                                       361  726                                                                       362  734                                                                       363  736                                                                       364  764                                                                       365  766                                                                       366  774                                                                       367  776                                                                       371  727                                                                       372  735                                                                       373  737                                                                       374  765                                                                       375  767                                                                       376  775                                                                       377  777                                                               ______________________________________                                    

                  TABLE 6                                                          ______________________________________                                                        CW      CW                                                                     even    odd                                                     Data           parity  parity                                                  ______________________________________                                         060            464     252                                                     070            465     225                                                     110            541     261                                                     120            544     144                                                     130            545     145                                                     150            561     161                                                     160            564     164                                                     170            565     165                                                     210            525     232                                                     220            461     272                                                     230            415     231                                                     250            431     221                                                     260            434     262                                                     270            435     223                                                     310            511     222                                                     320            514     114                                                     330            515     115                                                     350            531     131                                                     360            534     134                                                     370            535     135                                                     ______________________________________                                    

Again, the encoder 3 can be implemented with either combinatorial logic or sequential logic or can be implemented as a look-up table, e.g. a ROM. A person skilled in the art could also implement the encoding scheme in software without departing from the scope and spirit of the invention. In the preferred embodiment the encoder is implemented with combinatorial logic and the logic equations are listed in Table 7 where "abcdefgh" is the input byte, p is the parity of the previous codeword, and "ABCDEFGHJ" is the output codeword. An exclamation! indicates an inverted term. "A" and "a" are the most significant bits of the input byte and output codeword respectively.

                  TABLE 7                                                          ______________________________________                                         A =   a d !p + b d !f !g !h !p + a e !h !p + b e !f !g !h !p +                       c d !f !g !h !p + !a !c !d !e + !b !d f + !b !c f +                            a c h + a d g + a e g + a d h + a c g + a b e h +                              a f + !b !c !d !e.                                                       B =   a !b e p + b !c !d e p + !b c d p + a !b d p +                                 b c !d !e !g !h + b e g + b e h + a c !d !e !g !h +                            a e g + a e h + b e f + b d g + b d h +                                        c d g + c d h +b c f + a e f + a d g + a d h +                                 b d f + c d f + a c f + a d f.                                           C =   b c f + b e f + !a !b !d e + b c g + b e g +                                   b c h + b e h + !c !d e !f !g !h !p + b d +                                    b c e + !a !b c !d + !a !c d + a b !c !e.                                D =   !b c d !e !g !h p + a d f + b e f + b d f +                                    a !b !c d !e !g !h + !a b d !g !h + !a b e !g !h +                             !a c d !g !h !p + b c !d !e !f !h + !b !c !d !g h +                            !c !d !e h + !d !e g + !a !b !d h + !a !b !c h +                               !a !b !d g + !a !b !c g + a e f + c d f.                                 E =   a !b e !f !g !h p + a !b d !e !f !g !h +                                       a !b !d e !f !g + b !d !f !g !h p + c e + c !p +                               b c + !a !d !e f + b !d !e + c g + c h + c f + c !d.                     F =   a !c d e !f !h + a b d !f !h + g h + a b c !f !g !h +                          a b !f !g !h !p + !d !e !f !g !h + a c !f !g !h !p +                           !a !b !e !f !h p + !b !c !f !h p + b c !d !e h +                               a c !d !e h + !a !b !d !h + !a !b !c !h +                                      !c !d !e !h + d g + e g.                                                 G =   !a !c d + a !b !c e !f !g !h !p + b d + c d !p +                               !a d e + !a !b !d !e + !c !e f + a b !c !e +                                   d g + d h + d f.                                                         H =   !d !e f !g + a !b c d !f !g p + a !c !d !f !g p +                              !b !e !f !g p + !a !b !c !g + a c !d !e +                                      b c !d !e g + !a !b !d !g + !c !d !e !g + h.                             J =   b !d !e !f !g !h + e f + a c !d !f !g !h + e g +                               a !c !d !e + e h + b !c !d !e + e !p +                                         a !b !c !f !g !h !p + d e + !a e.                                        ______________________________________                                    

DETECTOR/DECODER

In the preferred embodiment of the present invention, the detector 4 is a pair of two state Viterbi detectors as shown in FIG. 8. The only difference between the minimum latency version and the error correcting version is that the error correcting version stores nine bits of the second codeword rather than three. In both versions, the path memories storing the first three bits of the second codeword are used to determine which set the second codeword belongs, and thus determines the parity of the first codeword.

As shown in Table 1, groups 1 and 2, which follow codewords with odd parity, have as the first three bits {0,0,1} and {0,1,0}, and groups 4 and 5, which follow codewords with even parity, have as the first three bits {1,0,0} and {1,0,1} . As shown in FIG. 5b and 5c, the survivor sequences stored in the path memories will be different by one bit in the odd interleave if the first two bits of the second codeword are {0,1} (as in group 2), and different by one bit in the even interleave if the first two bits of the second codeword are {1,0} or if the first three bits are {0,0,1} (as in groups 1, 4 and 5).

If the odd interleave is not merged, then the path memory is always chosen such that the parity of the first codeword is odd since the path memories storing the odd interleave will be different by one bit only when followed by a codeword from group 2. That is, the path memory storing four out of the nine bits of the first codeword is selected such that the parity of the first codeword is odd.

Since the path memories storing the even interleave (the five bits out of the nine bits) of the first codeword will be different by one bit when followed by codewords from groups 1, 4, and 5, the path memories storing the first bit of the second codeword are used to determine which group the second codeword belongs. If the path memories storing the first bit of the second codeword are "0", then the second codeword is from group 1 and the path memory storing the even interleave is selected such that the parity of the first codeword is odd. Otherwise, the second codeword is from group 4 or 5 and the path memory storing the even interleave is chosen such that the parity of the first codeword is even.

FIG. 8a, 8b and 8c show the cases where the second codeword is from group 1, group 4 and group 5 where the first three bits are {0, 0, 1} 90, {1, 0, 0} 94 and {1, 0, 1} 97 respectively. When the second codeword is from group 1, the path memories storing the first bit are all "0" 92. When the second codeword is from group 4, two of the path memories store a "0" bit and the other two a "1" bit 96. When the second codeword is from group 5, the path memories storing the first bit are all "1" 98.

A look-ahead (herein after LA₋₋ IN) signal is generated as the OR 136 of the first bit of the second codeword from the two middle path memories for the four path Viterbi detector as shown in FIG. 12. For the paired dicode Viterbi detector, the LA₋₋ IN signal is generated from the OR 325 of the two path memories storing the odd interleave as shown in FIG. 9b. When the second codeword is from group 1, the LA₋₋ IN signal is inactive since the path memories are all "0", and when the second codeword is from either group 4 or group 5, the LA₋₋ IN signal is active. The LA₋₋ IN signal selects the correct survivor sequence for the even interleave of the first codeword according to its parity as determined by which group the second codeword is from.

MINIMUM LATENCY DETECTOR/DECODER

Referring now to the preferred embodiment using a pair of dicode detectors as shown in FIG. 8d, before the detector processes any data samples the RST 110 signal clears counter 205 and flip-flops 208 and 209. Blocks 200 and 202 are sliding-threshold detectors which implement Viterbi detection for a 1-D channel. Block 200 receives the even numbered samples starting with sample 0. This sequence of samples will be referred to as interleave 0 or I0. Block 202 receives the odd numbered samples starting with sample 1. This sequence of samples will be referred to as interleave 1 or I1. Flip-flop 209 produces two complementary clock signals at half the frequency of the sample clock. CLK0 111 drives the I0 detector and CLK1 112 drives the I1 detector. With each data sample, counter 205 increments by one. When twelve samples have been processed, the detectors have received the nine samples of the first codeword and three additional look-ahead samples.

At this point 5₋₋ FROM₋₋ I0 114 is "1" and so five bits from I0 and four bits from I1 are presented to decoder 5 via MUXes 201 and 203 and on the next clock the first output byte is latched into register 207. On that same clock edge, comparator 206, which compares the output of counter 205 to 12 applied at line 119, re-loads counter 205 with 4 over line 118. Flip-flop 208 is toggled which sets 5₋₋ FROM₋₋ I1 116 to "1". The next codeword then will be assembled by taking five bits from I1 202 and four bits from I0 200. Operation continues in this way with MUX 201 alternating between selecting detector I0 200 and selecting detector I1 202 to supply the five-bit interleave of a codeword while MUX 203 selects the other detector to supply the four-bit interleave of a codeword.

Details of detectors 200 and 202, which are identical, are shown in FIG. 9a and FIG. 9b. In FIG. 9a blocks 300 to 308 make up the sliding-threshold detector and flip-flops 322₅ and 323₅ are the first bits of the path memories. The data samples have been normalized to take on peak values of -2 or +2 when uncorrupted by noise. Each data sample is compared with the upper threshold stored in register 304 and the lower threshold stored in register 305. These registers are initialized to 0 before the start of detection.

If the sample is greater than the upper threshold, then comparator 306 outputs a "1" which sets the upper threshold to the current sample value and the lower threshold to the current sample value minus 2. A "0" is shifted into lower path memory 323, and a "1" is shifted into the first stage of upper path memory 322₅. As a result, the path memories merge by loading upper path memory 322₀ to 322₄ with lower path memory 323₁ to 323₅ via MUXes 320 as shown in FIG. 9b.

If the sample is less than the lower threshold then comparator 307 outputs a "1" which sets the lower threshold to the current sample value and the upper threshold to the current sample value plus 2. A "0" is shifted into upper path memory 322, and a "1" is shifted into the first stage of lower path memory 323₅. As a result, the path memories merge by loading lower path memory 323₀ to 323₄ with upper path memory 322₁ to 322₅ via MUXes 321 as shown in FIG. 9b.

If the sample value is between the upper and lower thresholds then the thresholds are unchanged and a "0" is shifted into both path memories.

When the path memories are unmerged, the detector must select one of the two paths as the output. The selection circuitry is shown in FIG. 9b. Parity generator 327 generates the parity (modulo-2 sum) of the last four bits of upper path memory 322 if input ENA₋₋ 5₁₃ OUT 150 is "0" and generates the parity of the last five bits of upper path memory 322 if input ENA₋₋ 5₋₋ OUT 150 is "1". The selection logic, consisting of AND gate 329 and XOR gate 328, connects to the selection inputs of MUXes 324.

The PARITY₋₋ IN 152 and LA₋₋ IN 154 inputs of FIG. 9b are the PARITY₋₋ OUT 156 and LA₋₋ OUT 158 outputs from the other detector as shown in FIG. 8. The parity generators and selection logic implement the following path selection rules:

1. If the five-bit even interleave is merged and the four-bit odd interleave is not merged, then the four-bit path memory is chosen such that the parity of the first codeword (output codeword) is odd. This is done by selecting the upper path memory if the XOR 328 of the two generated parities is "1", i.e. selecting the path which generated odd parity, and by selecting the lower path memory if the XOR 328 of the two generated parities is "0".

2. If the four-bit odd interleave is merged and the five-bit even interleave is not merged and the LA₋₋ IN 154 is "0" (i.e. the second codeword is a group 1 codeword), then the five-bit path memory is chosen such that the parity of the first codeword (output codeword) is odd. This is done by selecting the upper path memory if the XOR 328 of the two generated parities is "1", i.e. selecting the path which generated overall odd parity, and by selecting the lower path memory if the XOR 328 of the two generated parities is "0".

3. If the four-bit odd interleave is merged and the five-bit even interleave is not merged and the LA₋₋ IN 154 bit is "1" (i.e. the second codeword is a group 4 or 5 codeword), then the five-bit path memory is chosen such that the parity of the first codeword (output it codeword) is even. This is done by selecting the upper path memory if the XOR 328 of the two generated parities is "0", i.e. selecting the path which generated overall even parity, and by selecting the lower path memory if the XOR 328 of the two generated parities is "1".

4. When both interleaves are merged then either path may be selected for output since both paths are identical.

MUX block 210 in FIG. 8 which contains MUXes 201 and 203 is detailed in FIG. 10. The outputs from detector I0 200 are received on lines 211_(x) and the outputs from detector I1 202 are received on lines 212_(x). MUXes 201 select the five-bit interleave and MUXes 203 select the four-bit interleave. The nine outputs are then interleaved into the output codeword "ABCDEFGHJ" which feeds decoder block 5 in FIG. 1.

The decoder 5 in FIG. 1 can be implemented with either combinatorial logic or sequential logic or can be implemented as a look-up table, e.g. a ROM. A person skilled in the art could also implement the decoding scheme in software without departing from the scope and spirit of the invention. In the preferred embodiment, it is implemented with combinatorial logic and the logic equations are listed in Table 8 where "ABCDEFGHJ" is the input codeword and "abcdefgh" is the output byte. An exclamation! indicates an inverted term. "A" and "a" are the most significant bits of the input codeword and output byte, respectively.

                  TABLE 8                                                          ______________________________________                                         a =   A B C F J + A B C D J + A B C !D E + !A B !C +                                 A C G + !B D F !J + !B E !F !J +                                               A !B !E F !J + A !B !C + !B G.                                           b =   !B E !F !J + A B !C J + A B !C !H +                                            A B !C !D E + !A !C D E !J + !C G +                                            !B C + !A !B + !B D F !J.                                                c =   A B C !D !F !G J + A D E F !G + A B C D E +                                    A C !D !E F !G !J + A B C !E !G !J +                                           A !B !C G + !A B !C G + A !B !C D F !J +                                       !B !D E F !G + E !G J + B E G + B D E F +                                      A !B !C E + !A B !C D E.                                                 d =   !B !C D E !J + !A !B D E !J +                                                  !B D E !F !J + A B G + B C G + E G.                                      e =   C !D E !F !G !J + B C D J + B C F J +                                          A !B !C D F !G !J + !A !B D E F !G !J +                                        !A !B !D E !F !G !J + A !B C !D !E !G !J +                                     A B !D E !G + B C.                                                       f =   A B !C !E F !G H !J + B C D E F + B !C D !E +                                  A B D E + D G + D J.                                                     g =   B C !E F + A B D F + A B !E F + F G + F J.                               h =   B !D !F H !J + !A B !E !F H + !B H J + E H J +                                 A !B C !D !E H + B C D F H + A !C !D !F H +                                    A !C !E !F H + !A B !C !F H + A B D !E H +                                     G H + F H J.                                                             ______________________________________                                    

ERROR CORRECTING DETECTOR/DECODER

As described above, the Viterbi sequence detector for the error correcting embodiment of the present invention is the same as the minimum latency embodiment except that nine bits of the second codeword are stored rather than three. FIGS. 11a and 11b show the Viterbi detectors with the extended path memories. The comparator 206 in FIG. 8d compares the counter 205 output to 18 which is applied at line 119. When the counter out put is 18, which is byte output time, counter 205 is loaded with 10 over line 118. The detectors 200 and 202 are shown in FIGS. 11a and 11b and are the same as FIGS. 9a and 9b with the exception that three bits of path memory are added to each detector as shown in FIG. 11a.

There are 512 different nine-bit sequences which can be output from the detector of which 276 are valid codewords. The remaining 236 sequences are invalid and indicate detection errors. The decoder maps each invalid sequence into a byte whose corresponding codeword differs from the invalid sequence in two contiguous positions within an interleave which is the most likely error that the PR4 detector can make given a typical magnetic disk read head response. This allows the decoder to correct many errors that are produced as a result of noisy samples. Table 9 lists the complete mapping of input sequences (SEQ) to output bytes (Data) in octal notation.

                  TABLE 9                                                          ______________________________________                                                 SEO  Data                                                              ______________________________________                                                 000  320                                                                       001  052                                                                       002  142                                                                       003  053                                                                       004  026                                                                       005  036                                                                       006  027                                                                       007  037                                                                       010  320                                                                       011  350                                                                       012  141                                                                       013  050                                                                       014  320                                                                       015  330                                                                       016  024                                                                       017  034                                                                       020  341                                                                       021  056                                                                       022  343                                                                       023  057                                                                       024  046                                                                       025  210                                                                       026  047                                                                       027  307                                                                       030  341                                                                       031  350                                                                       032  341                                                                       033  054                                                                       034  360                                                                       035  370                                                                       036  044                                                                       037  304                                                                       040  120                                                                       041  150                                                                       042  142                                                                       043  051                                                                       044  120                                                                       045  130                                                                       046  025                                                                       047  035                                                                       050  022                                                                       051  052                                                                       052  143                                                                       053  053                                                                       054  026                                                                       055  036                                                                       056  027                                                                       057  037                                                                       060  342                                                                       061  150                                                                       062  342                                                                       063  055                                                                       064  160                                                                       065  170                                                                       066  045                                                                       067  305                                                                       070  343                                                                       071  056                                                                       072  343                                                                       073  057                                                                       074  046                                                                       075  306                                                                       076  047                                                                       077  307                                                                       100  032                                                                       101  012                                                                       102  033                                                                       103  013                                                                       104  022                                                                       105  032                                                                       106  023                                                                       107  033                                                                       110  330                                                                       111  010                                                                       112  030                                                                       113  010                                                                       114  320                                                                       115  330                                                                       116  020                                                                       117  030                                                                       120  141                                                                       121  052                                                                       122  143                                                                       123  053                                                                       124  042                                                                       125  302                                                                       126  043                                                                       127  303                                                                       130  141                                                                       131  350                                                                       132  141                                                                       133  050                                                                       134  360                                                                       135  370                                                                       136  040                                                                       137  300                                                                       140  130                                                                       141  011                                                                       142  031                                                                       143  011                                                                       144  120                                                                       145  130                                                                       146  021                                                                       147  031                                                                       150  032                                                                       151  012                                                                       152  033                                                                       153  013                                                                       154  022                                                                       155  032                                                                       156  023                                                                       157  033                                                                       160  142                                                                       161  150                                                                       162  142                                                                       163  051                                                                       164  160                                                                       165  170                                                                       166  041                                                                       167  301                                                                       170  143                                                                       171  052                                                                       172  143                                                                       173  053                                                                       174  042                                                                       175  302                                                                       176  043                                                                       177  303                                                                       200  060                                                                       201  156                                                                       202  061                                                                       203  151                                                                       204  326                                                                       205  336                                                                       206  321                                                                       207  331                                                                       210  062                                                                       211  152                                                                       212  063                                                                       213  153                                                                       214  322                                                                       215  332                                                                       216  323                                                                       217  333                                                                       220  310                                                                       221  250                                                                       222  310                                                                       223  270                                                                       224  066                                                                       225  070                                                                       226  061                                                                       227  071                                                                       230  210                                                                       231  230                                                                       232  210                                                                       233  240                                                                       234  062                                                                       235  072                                                                       236  063                                                                       237  073                                                                       240  064                                                                       241  154                                                                       242  065                                                                       243  155                                                                       244  324                                                                       245  334                                                                       246  325                                                                       247  335                                                                       250  066                                                                       251  156                                                                       252  060                                                                       253  157                                                                       254  326                                                                       255  336                                                                       256  327                                                                       257  337                                                                       260  260                                                                       261  110                                                                       262  260                                                                       263  211                                                                       264  064                                                                       265  074                                                                       266  065                                                                       267  075                                                                       270  220                                                                       271  140                                                                       272  220                                                                       273  340                                                                       274  066                                                                       275  076                                                                       276  067                                                                       277  077                                                                       300  136                                                                       301  116                                                                       302  131                                                                       303  111                                                                       304  126                                                                       305  136                                                                       306  121                                                                       307  131                                                                       310  132                                                                       311  112                                                                       312  133                                                                       313  113                                                                       314  122                                                                       315  132                                                                       316  123                                                                       317  133                                                                       320  144                                                                       321  156                                                                       322  144                                                                       323  151                                                                       324  166                                                                       325  176                                                                       326  161                                                                       327  171                                                                       330  145                                                                       331  152                                                                       332  145                                                                       333  153                                                                       334  162                                                                       335  172                                                                       336  163                                                                       337  173                                                                       340  134                                                                       341  114                                                                       342  135                                                                       343  115                                                                       344  124                                                                       345  134                                                                       346  125                                                                       347  135                                                                       350  136                                                                       351  116                                                                       352  137                                                                       353  117                                                                       354  126                                                                       355  136                                                                       356  127                                                                       357  137                                                                       360  146                                                                       361  154                                                                       362  146                                                                       363  155                                                                       364  164                                                                       365  174                                                                       366  165                                                                       367  175                                                                       370  147                                                                       371  156                                                                       372  147                                                                       373  157                                                                       374  166                                                                       375  176                                                                       376  167                                                                       377  177                                                                       400  206                                                                       401  202                                                                       402  207                                                                       403  203                                                                       404  002                                                                       405  206                                                                       406  003                                                                       407  207                                                                       410  230                                                                       411  200                                                                       412  204                                                                       413  200                                                                       414  000                                                                       415  230                                                                       416  000                                                                       417  204                                                                       420  241                                                                       421  102                                                                       422  243                                                                       423  103                                                                       424  006                                                                       425  106                                                                       426  007                                                                       427  107                                                                       430  241                                                                       431  250                                                                       432  241                                                                       433  100                                                                       434  260                                                                       435  270                                                                       436  004                                                                       437  104                                                                       440  205                                                                       441  201                                                                       442  205                                                                       443  201                                                                       444  001                                                                       445  205                                                                       446  001                                                                       447  205                                                                       450  206                                                                       451  202                                                                       452  207                                                                       453  203                                                                       454  002                                                                       455  206                                                                       456  003                                                                       457  207                                                                       460  242                                                                       461  220                                                                       462  242                                                                       463  101                                                                       464  060                                                                       465  070                                                                       466  005                                                                       467  105                                                                       470  243                                                                       471  102                                                                       472  243                                                                       473  103                                                                       474  006                                                                       475  106                                                                       476  007                                                                       477  107                                                                       500  036                                                                       501  016                                                                       502  037                                                                       503  017                                                                       504  026                                                                       505  036                                                                       506  027                                                                       507  037                                                                       510  330                                                                       511  310                                                                       512  034                                                                       513  014                                                                       514  320                                                                       515  330                                                                       516  024                                                                       517  034                                                                       520  341                                                                       521  056                                                                       522  343                                                                       523  057                                                                       524  046                                                                       525  210                                                                       526  047                                                                       527  307                                                                       530  341                                                                       531  350                                                                       532  341                                                                       533  054                                                                       534  360                                                                       535  370                                                                       536  044                                                                       537  304                                                                       540  130                                                                       541  110                                                                       542  035                                                                       543  015                                                                       544  120                                                                       545  130                                                                       546  025                                                                       547  035                                                                       550  036                                                                       551  016                                                                       552  037                                                                       553  017                                                                       554  026                                                                       555  036                                                                       556  027                                                                       557  037                                                                       560  342                                                                       561  150                                                                       562  342                                                                       563  055                                                                       564  160                                                                       565  170                                                                       566  045                                                                       567  305                                                                       570  343                                                                       571  056                                                                       572  343                                                                       573  057                                                                       574  046                                                                       575  306                                                                       576  047                                                                       577  307                                                                       600  236                                                                       601  216                                                                       602  231                                                                       603  217                                                                       604  226                                                                       605  236                                                                       606  221                                                                       607  231                                                                       610  232                                                                       611  212                                                                       612  233                                                                       613  213                                                                       614  222                                                                       615  232                                                                       616  223                                                                       617  233                                                                       620  244                                                                       621  256                 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                           653  217                                                                       654  226                                                                       655  236                                                                       656  227                                                                       657  237                                                                       660  246                                                                       661  254                                                                       662  246                                                                       663  255                                                                       664  264                                                                       665  274                                                                       666  265                                                                       667  275                  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704  326                                                                       705  336                                                                       706  321                                                                       707  331                                                                       710  332                                                                       711  312                                                                       712  333                                                                       713  313                                                                       714  322                                                                       715  332                                                                       716  323                                                                       717  333                                                                       720  344                                                                       721  356                                                                       722  344                                                                       723  351                                                                       724  366                                                                       725  376                                                                       726  361                                                                       727  371                                                                       730  345                                                                       731  352                                                                       732  345                                                                       733  353                                                                       734  362                                                                       735  372                                                                       736  363                                                                       737  373                                                                       740  334                                                                       741  314                                                                       742  335                                                                       743  315                                                                       744  324                                                                       745  334                                                                       746  325                                                                       747  335                                                                       750  336                                                                       751  316                                                                       752  337                                                                       753  317                                                                       754  326                                                                       755  336                                                                       756  327                                                                       757  337                                                                       760  346                                                                       761  354                                                                       762  346                                                                       763  355                                                                       764  364                                                                       765  374                                                                       766  365                                                                       767  375                                                                       770  347                                                                       771  356                                                                       772  347                                                                       773  357                                                                       774  366                                                                       775  376                                                                       776  367                                                                       777  377                                                               ______________________________________                                    

The decoder can be implemented with either combinatorial logic or sequential logic or can be implemented as a look-up table, e.g. a ROM. A person skilled in the art could also implement the decoding scheme in software without departing from the scope and spirit if the invention. In the preferred embodiment it is implemented with combinatorial logic and the logic equations are listed in Table 10 where "ABCDEFGHJ" is the input codeword and "abcdefgh" is the output byte. An exclamation! indicates an inverted term. "A" and "a" are the most significant bits of the input codeword and output byte, respectively.

                  TABLE 10                                                         ______________________________________                                         a =  !B !C !D F !H J + !B !D !E F !G !H !J + !B C !D E G J +                        !B C E F G J + A !D E F !H + !B C E G H J +                                    A !C !G !J + !A !B !D F G !H + A C !D F !H +                                   !A !B E G H J + !A !B !D E G J + !C E !G !J +                                  !B !D E F !H J + !B !C !D !G !H !J + !A !B E F G J +                           B !C !E G + A E !G !J + B !C E !G H + A !C !E J +                              A B + B !C !D E !G + A !C D !F !G !H.                                     b =  C !D F G !H + C D !F !H !J + C !D E F !H +                                     C D E !F !H + !A !B !C !F !G !J + !B D E F G J +                               !A !C D !F !G !H J + C !D F !H !J + !A !B !C !D F !H +                         !A !B D !F G !H + !B E G H J + A !B !C E H J +                                 !A !B !C !G H !J + A !B !C !D E !F J +                                         A !B !C D E F J + !A !B E !G !J + !A B !E G +                                  !A B !E J + A C !D F !H + !A !D E !F !G !J +                                   C E !G !J + A C D !F !H + !A B D F !G J +                                      B C + !A C E G J.                                                         c =  B !D E !F J + D E !F !G !J + B D E F J +                                       B !D E H J + !B !D E F !H + !A E G !J +                                        !A !B !C !G J + !A !B !C !G H + !B E !G !J +                                   !A B !C !E !G + D E !F G !H + C E !J + A B E +                                 C E !G + B E G.                                                           d =  A !B !C D E !F !H J + !A B !C !D E !F H J +                                    !A B !C !D E F !H J + C !E !J + !D !E F !G !H !J +                             !A B !C D !J + !D E F G !H + D E !F G !H +                                     !A !E !H !J + B !E !J + B G + C !E G + !A !E G +                               !D F G !H J.                                                              e =  A C !F !H J + !A !F !G J + !A !C !F !H J +                                     !A B !C !D E !G !J + A !D !E F !G !H !J + C !E !G +                            A B !E !G + D E !F G !H J + !D F G !H J + B G J +                              !A !E J + !A !B !G J + !D E F !H J + C !E J +                                  C !G J + A B J.                                                           f =  A C D !E F + A !C D !E !G !J + !A !C !D !F G !H !J +                           A C !D !F !G !H J + A !D !E !F G !H J +                                        !C !D !E !F G !H J + A !D !E !F !G !H !J +                                     A !C D !E G J + A !C !D E !F G !H + A D E F G +                                A !B C G H + A C !D !F G !H !J + A !B E G H +                                  !B !C G H J + !A !B !C E H J + !A !B !C !D E !F !G J +                         !A !B !C G H + B C E !G !J + A !B C H J +                                      A !B !E !G H !J + !A !B !C D E F J + B C !F !H +                               B !E !F !H J + A B E !G !J + !A !C D F G +                                     A C D F J + A B !F !H + B C D + B D !E !H +                                    A B !C !E !F !G J + B D !E J + B D !E !F +                                     A B D + B D G.                                                            g =  !A !B !C !F !G H !J + A D F + C D F + D !E F J +                               A !C !D !E !F !G J + B C !D !H J + !A C !D !F !H J +                           A !C !D !F !H J + A !D !E !F !H + D !E F !H +                                  C !D !E !F !H + !D !F G !H !J + !D !E !F !H J +                                !B !D !F H + !B D E !G !J + !B !D !F !G J +                                    C D E !G !J + A D E !G !J + !B D F + B F G +                                   B !D !E F + B C F J + A B F J.                                            h =  A !E !F H + C !F H J + !A !C !D !E F !G H !J +                                 A B F H + A B H J + C !E !F H + B !E H J +                                     !B !F H J + !B D F H + B C F H + !B !D E !G !J +                               C E F !G !J + A E F !G !J + !B E F !G !J +                                     !A !B C D !E !F !G J + A !B !C D !E !F + B G H +                               !F G H + B !E !F H + D !F H J.                                            ______________________________________                                    

1-D² VITERBI DETECTOR

An alternative embodiment of the detector 4 is to use a four state 1-D² Viterbi detector rather than a pair of two state 1-D sliding threshold Viterbi detectors. Except for the Viterbi algorithm, the operation of the encoding and decoding scheme is exactly as described above. FIG. 12 shows a circuit diagram for the 1-D² Viterbi detector 4 embodiment.

Block 120 implements the 1-D² Viterbi algorithm and the path memories 122 store nine bits 126 of a first codeword and three bits 124 of a second (following) codeword. To understand the invention, each path memory represents a state of the trellis diagram as shown in FIG. 5. Parity Generator 128 generates the parity of the five bit even interleave of the first codeword, and Parity Generator 130 generates the parity for the four bit odd interleave. Multiplexors 732 and 134 output the first codeword 14 by selecting the path memory based on the parity as determined by the second codeword as described above. The first look-ahead bit of the second codeword, generated by OR gate 136, is used for selecting the parity of the first codeword when the five bit even interleave has not merged.

What has been disclosed and described is a novel method for encoding, detecting decoding data in a PR class-IV digital magnetic recording channel. By utilizing parity information from the encoded codewords, the interleave constraint on the maximum number of zeros in conventional systems is avoided. As a result, the path memory and latency of the Viterbi detector is minimized. A further benefit is the ability to correct for certain errors caused when a "1" bit is erased due to noise in the system.

Those skilled in the art can appreciate that the aspects of the invention can be achieved in various differing embodiments, in circuitry or in software, without departing from the spirit and scope of the invention. Further, the application of the present invention to RLL coding schemes, other than those disclosed, is apparent. The specific RLL coding constraints disclosed is intended to be illustrative and is not meant to limit the scope of the invention as appropriately construed by the following claims. 

We claim:
 1. An apparatus for processing an input stream of pulse samples in a PRML communication channel, comprising:(a) a receiver connected to receive the input stream of pulse samples; (b) at least one sequence detector, connected to receive the pulse samples, for detecting a digital sequence of binary data from the pulse samples, comprising a plurality of path memories, the path memories comprising a plurality of memory elements, each memory element storing one bit of the digital sequence; (c) at least one parity generator for generating a parity signal having two states, active and inactive, responsive to the number of "1" bits stored in at least two of the memory elements of at least one of the path memories of the sequence detector: (d) at least one path selector for selecting one of two of the path memories out of the plurality of path memories of the sequence detector, the path selector using the parity signal for selecting the path memory; and (e) at least one look ahead generator for generating a look ahead signal having two states, active and inactive, responsive to the bits stored in at least one of the memory elements of at least two of the path memories of the sequence detector, the path selector using the look ahead signal for selecting the path memory.
 2. The apparatus as recited in claim 1, wherein:the digital sequence comprising a plurality of codewords wherein each codeword comprises a predetermined k number of bits; and the sequence detector storing a first codeword and at least part of a second codeword in the plurality of path memories; the parity signal being generated from the memory elements of the path memories storing at least part of the first codeword; and the look ahead signal being generated from the memory elements of the path memories storing at least part of the second codeword; and the path selector selecting the path memory storing at least part of the first codeword.
 3. The apparatus as recited in claim 2, wherein:the sequence detector comprises four path memories comprising n+k memory elements each, wherein n is the number of bits of the second codeword stored in the path memories; and the path memories storing two survivor sequences of k bits, different in only one bit, wherein one of the survivor sequences storing the first codeword; and the path selector selecting the path memory storing the survivor sequence comprising the first codeword.
 4. The apparatus as recited in claim 3, wherein:the path memories storing an even and odd interleave subsequence, the odd interleave consisting only of odd bit positions and the even interleave consisting of even bit positions; and the at least one parity generator comprising a first and second parity generator; and the first parity generator responsive to the memory elements storing the odd interleave of the two survivor sequences of k bits; and the second parity generator responsive to the memory elements storing the even interleave of the two survivor sequences of k bits; and the at least one path selector comprising a first and second path selector; and the first path selector responsive to the first parity generator and for selecting the path memory storing the odd interleave of the survivor sequence comprising the first codeword; and the second path selector responsive to the second parity generator and for selecting the path memory storing the even interleave of the survivor sequence comprising the first codeword; and the look ahead signal responsive to at least one of the n memory elements of at least two of the path memories.
 5. The apparatus as recited in claim 2, wherein:the input stream of pulse samples is divided into an even and odd interleave; and the first codeword comprising a first and second part wherein each part comprising at least one bit; and the sequence detectors comprise two sequence detectors, each sequence detector comprising two path memories, each sequence detector processing one of the interleaves; and the path memories of one of the sequence detectors storing a single survivor sequence, the single survivor sequence comprising the first part of the first codeword; and the path memories of the other sequence detector storing two survivor sequences, different in one bit, wherein one of the two survivor sequences comprising the second part of the first codeword; and the path selector selecting the path memory storing the survivor sequence comprising the second part of the first codeword; and the look ahead signal responsive to the sequence detector comprising the path memories storing the single survivor sequence.
 6. An apparatus for processing an input stream of pulse samples in a PRML communication channel, comprising:(a) a receiver connected to receive the input stream of pulse samples; (b) at least one sequence detector, connected to receive the pulse samples, for detecting a digital sequence of binary data from the pulse samples, comprising a plurality of path memories, the path memories comprising a plurality of memory elements, each memory element storing one bit of the digital sequence; (c) at least one parity generator for generating a parity signal having two states, active and inactive, responsive to the number of "1" bits stored in at least two of the memory elements of at least one of the path memories of the sequence detector; and (d) at least one path selector for selecting one of two of the path memories out of the plurality of path memories of the sequence detector, the path selector using the parity signal for selecting the path memory,wherein: the digital sequence comprising a plurality of codewords wherein each codeword comprises a predetermined k number of bits; the sequence detector storing a first codeword and at least part of a second codeword in the plurality of path memories; the parity signal being generated from the memory elements of the path memories storing at least part of the first codeword; and the path selector selecting the path memory such that the parity of bits out of the digital sequence detected matches the parity of the first codeword.
 7. The apparatus as recited in claim 6, further comprising a decoder for decoding the codewords into a stream of datawords comprising a predetermined j number of bits.
 8. The apparatus as recited in claim 7, wherein j is 8 and k is
 9. 9. The apparatus as recited in claim 7, wherein:the input stream of pulse samples is divided into an even and odd interleave; and k is greater than j and there are 2^(k) sequence words comprising 2^(j) codewords and 2^(k-j) errorwords, wherein the codewords decode into 2^(j) datawords; and the errorwords decode into datawords having corresponding codewords different from the errorwords in two contiguous bit positions within either interleave.
 10. The apparatus as recited in claim 6, wherein:the sequence detector comprises four path memories comprising n+k memory elements each, wherein n is the number of bits of the second codeword stored in the path memories; and the path memories storing two survivor sequences of k bits different in only one bit, wherein one of the survivor sequence storing the first codeword; and the path selector selecting the path memory storing the survivor sequence comprising the first codeword.
 11. The apparatus as recited in claim 10, wherein:the path memories storing an even and odd interleave subsequence, the odd interleave consisting only of odd bit positions and the even interleave consisting of even bit positions; and the at least one parity generator comprising a first and second parity generator; and the first parity generator responsive to the memory elements storing the odd interleave of the two survivor sequences of k bits; and the second parity generator responsive to the memory elements storing the even interleave of the two survivor sequences of k bits; and the at least one path selector comprising a first and second path selector; and the first path selector responsive to the first parity generator and for selecting the path memory storing the odd interleave of the survivor sequence comprising the first codeword; and the second path selector responsive to the second parity generator and for selecting the path memory storing the even interleave of the survivor sequence comprising the first codeword.
 12. The apparatus as recited in claim 6, wherein:the input stream of pulse samples is divided into an even and odd interleave; and the first codeword comprising a first and second part wherein each part comprising at least one bit; and the sequence detectors comprise two sequence detectors, each sequence detector comprising two path memories, each sequence detector processing one of the interleaves; and the two path memories of one of the sequence detectors storing a single survivor sequence, the single survivor sequence storing the first part of the first codeword; and the two path memories of the other sequence detector storing two corresponding survivor sequences, different in one pit, wherein one of the two survivor sequences comprising the second part of the first codeword; and the path selectors selecting the path memory storing the survivor sequence comprising the second part of the first codeword.
 13. An apparatus for processing an input stream of pulse samples in a PRML communication channel, comprising:(a) a receiver connected to receive the input stream of pulse samples; (b) at least one sequence detector, connected to receive the pulse samples, for detecting a digital sequence of binary data from the pulse samples, comprising a plurality of path memories, the path memories comprising a plurality of memory elements, each memory element storing one bit of the digital sequence; (c) at least one parity generator for generating a parity signal having two states, active and inactive, responsive to the number of "1" bits stored in at least two of the memory elements of at least one of the path memories of the sequence detector; and (d) at least one path selector for selecting one of two of the path memories out of the plurality of path memories of the sequence detector, the path selector using the parity signal for selecting the path memory,wherein: the input stream of pulse samples is divided into an even and odd interleave; and the sequence detectors comprise two sequence detectors, each sequence detector comprising two path memories, each sequence detector processing one of the interleave.
 14. The apparatus as recited in claim 13, wherein:the sequence detectors are sliding threshold sequence detectors; and the PRML communication channel is a PR4 communication channel.
 15. A method for processing an input stream of pulse samples in a PRML communication channel, comprising the steps of:(a) receiving an input stream of pulse samples; (b) detecting a digital sequence of binary data from the pulse samples utilizing at least one sequence detector comprising a plurality of path memories, the path memories comprising a plurality of memory elements, each memory element storing one bit of the digital sequence; (c) generating at least one parity signal having two states, active and inactive, responsive to the number of "1" bits stored in at least two of the memory elements of at least one of the path memories of the sequence detector; and (d) utilizing the part signal for selecting one of two of the path memories out of the plurality of path memories of the sequence detector;wherein: the digital sequence comprising a plurality of codewords wherein each codeword comprises a predetermined k number of bits; the sequence detector storing a first codeword and at least part of a second codeword in the plurality of path memories; the at least one parity signal being generated from the memory elements of the path memories storing at least part of the first codeword; and the path memory is selected in step (d) such that the parity of k bits out of the digital sequence detected matches the parity of the first codeword.
 16. The method as recited in claim 15, further comprising the step of decoding the codewords into a stream of datawords comprising a predetermined j number of bits.
 17. The method as recited in claim 16, wherein j is 8 and k is
 9. 18. The method as recited in claim 16, wherein:the input stream of pulse samples is divided into an even and odd interleave; and k is greater than j; and there are 2^(k) sequence words comprising 2^(j) codewords and 2^(k-j) errorwords, wherein the codewords decode into 2^(j) datawords; and the errorwords decode into datawords having corresponding codewords different from the errorwords in two contiguous bit positions within either interleaved.
 19. The method as recited in claim 15, wherein the sequence detector comprises four path memories.
 20. The method as recited in claim 15, wherein:the sequence detector comprises four path memories comprising n+k memory elements each, wherein n is the number of bits of the second codeword stored in the path memories; and the path memories storing two survivor sequences of k bits different in only one bit, wherein one of the survivor sequence storing the first codeword; and the path memory selected in step (d) storing the survivor sequence comprising the first codeword.
 21. The apparatus as recited in claim 20, wherein:the path memories storing an even and odd interleave subsequence, the odd interleave consisting only of odd bit positions and the even interleave consisting of eve bit positions; and the at least one parity signal comprising a first and second parity signal; and the first parity signal responsive to the memory elements storing the odd interleave of the two survivor sequences of k bits; and the second parity signal responsive to the memory elements storing the even interleave of the two survivor sequences of k bits; and selecting the path memory in step (d) comprises the steps of:utilizing the first parity signal for selecting the path memory storing the odd interleave of the survivor sequence comprising the first codeword; and utilizing the second parity signal for selecting the path memory storing the even interleave of the survivor sequence comprising the first codeword.
 22. The method as recited in claim 15, wherein:the input stream of pulse samples is divided into an even and odd interleave; and the sequence detectors comprise two sequence detectors, each sequence detector comprising two path memories, each sequence detector processing one of the interleaves.
 23. The method as recited in claim 22, wherein:the sequence detectors are sliding threshold sequence detectors; and the PRML communication channel is a PR4 communication channel.
 24. The method as recited in claim 15, wherein:the input stream of pulse samples is divided into an even and odd interleave; and the first codeword comprising a first and second part wherein each part comprising at least one bit; and the sequence detectors comprise two sequence detectors, each sequence detector comprising two path memories, each sequence detector processing one of the interleaves; and the two path memories of one of the sequence detectors storing a single survivor sequence, the single survivor sequence storing the first part of the first codeword; and the two path memories of the other sequence detector storing two corresponding survivor sequences, different in one pit, wherein one of the two survivor sequences comprising the second part of the first codeword; and the path memory selected in step (d) storing the survivor sequence comprising the second part of the first codeword. 